Whilst many algorithms exist for interferometric image reconstruction, there are not yet algorithms for polarimetric interferometric image reconstruction. The polarisation state of light contains critical information otherwise uncaptured by standard, unpolarised interferometry, and many major facilities are now looking towards fully leveraging this information to broaden the observational reach of new and existing instruments. Polarimetric image reconstruction has additional challenges compared to unpolarised image reconstruction, as reconstructions of polarised images (Stokes I, Q and U) are spatial maps of vector components. As such, they need to individually and collectively display physically realistic and mutually consistent scattering physics. Within the present work, we demonstrate that a two-stage machine learning framework (a convolutional neural network (CNN) + iterative fitting) can be used to successfully perform polarimetric image reconstruction, whilst satisfying these challenging regularisation requirements. Using a custom set of MCFOST radiative transfer models, we train a convolutional neural network to learn the mapping between polarised images and interferometric polarimetric observables. We then deploy an iterative fitting mechanism inspired by the Deep Image Prior, which iteratively improves the fit of polarimetric observables with cognisance of observational errors. In particular, the improvement provided by iterative fitting also results in the reconstruction of physically meaningful image structures that were missing from the original CNN image reconstruction. Our results suggest that this two-stage framework is a powerful tool for performing image reconstruction with complex regularisation constraints - in both polarimetric and non-polarimetric contexts. Here we briefly report our algorithm and initial results.
We present upgrades to the visible-light (600nm to 800 nm) high-contrast imaging polarimeter, VAMPIRES, installed on the Subaru Coronagraphic Extreme Adaptive Optics instrument (SCExAO). These upgrades included low read noise, high dynamic range qCMOS detectors, a novel dichroic-based multiband imaging optic for spectral differential imaging, a suite of classic Lyot coronagraphs, and an achromatic fast polarization modulator. VAMPIRES is capable of diffraction-limited imaging (17 mas to 22 mas angular resolution) and achieves Strehl ratios up to ∼60% with 5σ coronagraphic contrast of 10-4 at 0.1′′ and 10-6 at >0.4′′. We highlight commissioning results from HD 169142, R Aqr, Neptune, and HD 1160B.
The Visible Aperture Masking Polarimetric Imager for Resolved Exoplanetary Structures (VAMPIRES) is a visible light instrument on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system. In a previous work, the instrumental polarization (IP) and crosstalk of VAMPIRES was characterized from measurements with a polarized internal calibration source. In this work, we use unpolarized standard star observations to estimate the polarization of Subaru’s Nasmyth mirror (M3) and use polarized standard star observations to quantify the overall system Mueller matrix model’s accuracy. We present an instrument matrix inversion routine that has returned stable results for all tested on-sky sources. With the current 675nm Mueller matrix model of VAMPIRES, the median residuals when retrieving the on-sky degree of linear polarization (DoLP) and angle of linear polarization (AoLP) of three polarized standards were 0.27% and 3.57% respectively.
The Visible Aperture Masking Polarimetric Imager for Resolved Exoplanetary Structures (VAMPIRES) is a visible light instrument on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system. To characterize the Instrumental Polarization (IP) and crosstalk of the system, measurements with a polarized internal calibration source were taken to provide diattenuation and retardance estimates of components downstream of the telescope mirrors. Using Markov Chain Monte Carlo (MCMC) simulations, we developed a Mueller matrix model of VAMPIRES in five wavelength bands. The process of deriving the model revealed degeneracies involving multiple possible sets of values for highly correlated instrument parameters. It also revealed a significant difference in the EM gain ratio between VAMPIRES’ two cameras and the highly non-ideal behavior of the Ferroelectric Liquid Crystal (FLC). Observations of unpolarized and polarized standard stars from two separate nights have been collected and are currently being analyzed to provide an estimate of M3’s diattenuation and retardance as well, and the polarized standard stars in particular will be used as a final metric to quantify the instrument model’s accuracy. The current Mueller matrix model has already been applied to preliminary data analysis for the circumstellar disk HD34700 and has helped confirm that an observed 17° phase shift in differential visibilities for Mu Cephei is astrophysical in nature. The non-ideal behaviour of certain components highlights the need for a standard calibration procedure that should be performed regularly to track changes in VAMPIRES’ polarization qualities. Our laboratory and on-sky data collection and analysis procedures will be refined to be re-used to calibrate future versions of VAMPIRES, including its most recent upgrade in 2023.
We present recent laboratory results demonstrating high-contrast coronagraphy for future space-based large segmented telescopes such as the Large UV, Optical, IR telescope (LUVOIR) mission concept studied by NASA. The High-contrast Imager for Complex Aperture Telescopes (HiCAT) testbed aims to implement a system-level hardware demonstration for segmented aperture coronagraphs with wavefront control. The telescope hardware simulator employs a segmented deformable mirror with 36 hexagonal segments that can be controlled in piston, tip, and tilt. In addition, two continuous deformable mirrors are used for high-order wavefront sensing and control. The low-order sensing subsystem includes a dedicated tip-tilt stage, a coronagraphic target acquisition camera, and a Zernike wavefront sensor that is used to measure low-order aberration drifts. We explore the performance of a segmented aperture coronagraph both in “static” operations (limited by natural drifts and instabilities) and in “dynamic” operations (in the presence of artificial wavefront drifts added to the deformable mirrors), and discuss the estimation and control strategies used to reach and maintain the dark zone contrast. We summarize experimental results that quantify the performance of the testbed in terms of contrast, inner/outer working angle and bandpass, and analyze limiting factors by comparing against our end-to-end models.
High dispersion coronagraphy (HDC) is a technique that combines high contrast imaging techniques with high spectral resolution spectroscopy to directly characterize exoplanets and provide key information such as chemical composition, temperature, and rotational velocity. A consequence of adaptive optics systems used in direct imaging is the formation of residual bright spots of star lights, called speckles, in the final image. Due to the large difference in brightness between host stars and their planets, these speckles can easily obscure potential exoplanets. In a previous demonstration, it was shown that using monochromatic light and a fiber injection unit (FIU), simulated exoplanet light can be directed to a high-resolution spectrograph. The method had speckle suppression that exceeding conventional image-based speckle nulling. With a previous Kalman filter estimator implementation, we found that with the implementation of the algorithm, speckle suppression was even more stable and outperformed traditional speckle nulling. In this update to the estimator, progress has been made in terms of a new filter design, and better estimates of the physical parameters in the laboratory, resulting in a higher speckle nulling performance.
We discuss the use of parametric phase-diverse phase retrieval to characterize and optimize the transmitted wavefront of a high-contrast apodized pupil coronagraph with and without an apodizer. We apply our method to correct the transmitted wavefront of the HiCAT (High contrast imager for Complex Aperture Telescopes) coronagraphic testbed. This correction requires a series of calibration steps, which we describe. The correction improves the system wavefront from 16 nm RMS to 3.0 nm RMS for the case where a uniform circular aperture is in place. We further measure the wavefront with the apodizer in place to be 11.7 nm RMS. Improvement to the apodized pupil phase retrieval process is necessary before a correction based on this measurement can be applied.
A major science goal of future, large-aperture, optical space telescopes is to directly image and spectroscopically analyze reflected light from potentially habitable exoplanets. To accomplish this, the optical system must suppress diffracted light from the star to reveal point sources approximately ten orders of magnitude fainter than the host star at small angular separation. Coronagraphs with microdot apodizers achieve the theoretical performance needed to image Earth-like planets with a range of possible telescope designs, including those with obscured and segmented pupils. A test microdot apodizer with various bulk patterns (step functions, gradients, and sinusoids) and 4 different dot sizes (3 μm, 5 μm, 7 μm, and 10 μm) made of small chrome squares on anti-reflective glass was characterized with microscopy, optical laser interferometry, as well as transmission and reflectance measurements at wavelengths λ=600 nm and λ=800 nm. Microscopy revealed the microdots were fabricated to high precision. Results from laser interferometry showed that the phase shifts observed in reflection vary with the local microdot fill factor. This effect is not explained purely by interference between reflected fields from the chrome and glass portions. Transmission measurements showed that microdot fill factor and transmission were linearly related for dot sizes ≥5 μm. However, anomalously high transmittance was measured when the dot size is <5× the wavelength (i.e. ∼3 μm) and the fill factor is approximately 50%, where the microdot pattern becomes periodic. The transmission excess is not as prominent in the case of larger dot sizes suggesting that it is likely to be caused by the interaction between the incident field and electronic resonances in the surface of the metallic microdots. We used our empirical models of the microdot apodizers to optimize a second generation of reflective apodizer designs, specifically for demonstrating end-to-end instrumentation for planet characterization at Caltech’s High Contrast Spectroscopy Testbed for Segmented Telescopes (HCST), and confirmed that the amplitude and phase of the reflected beam closely matches the ideal wavefront.
Imaging Earth-like exoplanets with future space telescopes will require a coronagraph instrument that is capable of creating a dark zone in the starlight at the image plane that is ten orders of magnitude fainter than the off-axis image of the host star. What is more, the coronagraph must simultaneously provide a stable dark zone and high throughput over the angular separations that correspond to habitable zones around nearby Sun-like stars (~10-100 milliarcseconds). Since the pupils of most large-aperture space telescope architectures are likely to be obstructed by secondary mirrors, spider support structures, and gaps between mirror segments, the coronagraph optics must also be specially tailored to passively suppress starlight diffracted from the obstructions and discontinuities in the telescope pupil. Here, we demonstrate an apodized vortex coronagraph optimized for an off-axis segmented telescope on the new High Contrast Spectroscopy Testbed for Segmented Telescopes (HCST) at Caltech. The coronagraph consists of a microdot apodizer, a liquid crystal vortex phase mask in the focal plane, and a Lyot stop. The microdot apodizer is an AR-coated glass window with 10um gold microdots to be used in reflection around lambda=800nm. We describe the HCST optical system; the apodizer optimization, fabrication, and metrology procedures; and present end-to-end testbed results of the coronagraph coupled with a 32x32 Boston Micromachines deformable mirror for wavefront control. We aim to achieve a dark zone 10^-7 times fainter than the simulated host star over a wavelength range of 800±40nm in Spring 2018. Finally, we will outline future plans to demonstrate coronagraph concepts for centrally obscured telescopes.
The High Contrast Spectroscopy Testbed for Segmented Telescopes (HCST) at Caltech is aimed at filling gaps in technology for future exoplanet imagers and providing the U.S. community with an academic facility to test components and techniques for high contrast imaging with future segmented ground-based telescope (TMT, E-ELT) and space-based telescopes (HabEx, LUVOIR). The HCST will be able to simulate segmented telescope geometries up to 1021 hexagonal segments and time-varying external wavefront disturbances. It also contains a wavefront corrector module based on two deformable mirrors followed by a classical 3-plane single-stage corona- graph (entrance apodizer, focal-plane mask, Lyot stop) and a science instrument. The back-end instrument will consist of an imaging detector and a high-resolution spectrograph, which is a unique feature of the HCST. The spectrograph instrument will utilize spectral information to characterize simulated planets at the photon-noise limit, measure the chromaticity of new optimized coronagraph and wavefront control concepts, and test the overall scientific functions of high-resolution spectrographs on future segmented telescopes.
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